A system for transmitting motion in a bicycle includes a chain extending between toothed wheels associated with the crank axle and associated with the hub of the rear wheel. When there is more than one toothed wheel at at least one of the crank axle and the hub of the rear wheel, a gearshift is provided that includes a chain guide element, also known as derailleur, movable to displace the chain among the toothed wheels in order to change the gear ratio.
In the case of an electronically servo-assisted gearshift, for example in U.S. patent application Ser. No. 12/133,625, the displacement of the derailleur takes place through an electromechanical actuator, typically including an electric motor and an articulated parallelogram system. Control electronics automatically drive the actuator, based upon one or more detected variables such as the travel speed, the cadence of revolution of the cranks, the torque applied to the cranks, the slope of the travel terrain, the heart rate of the cyclist and similar variables, and/or based upon commands manually entered by the cyclist through suitable command members, such as levers or buttons.